bug#38003: date --date=-1month gives same month today

[Ilja Honkonen]

> Subtracting 1 month from October 31st results in September 31st.
> Since the date doesn't exist, it is normalized:
> September 31st is "one day after September 30th", which
> results in October 1st.

Thanks for explanation. To me it makes more sense that going one month 
back from 31 oct, or last day of oct, would give last day of sep instead 
of first day of oct. But now that I'm aware of this feature I can work 
around it if needed.
Ilja